Emergence Day is the day that the Locust launched their attack on planet Sera. Also known as E-Day, this day marks the beginning of the COG-Locust War. Tons of locust appeared on Sera's surface through Emergence Holes, and the slaughtering of humans commenced. It is still unknown how the Locust came to be, only that they came from underground. Emergence day came shortly after the end of the Pendulum Wars, consequently humanity was completely unprepared and lacked the coordination and supplies to fight back the Locust threat. A quarter of the human race was wiped out rapidly by the newly surfaced threat.
Emergence day is also known as the day that Gears of War was officially released in the shops in North America.
One possible reason that Emergence Day occurred was that the Locust are desperate for territory due to the Lambent Locust invading the Hollow, as seen evident in Gears of War 2. However, the fact Locusts and Lambent worked together in the Lethia Imulsion Facility to repel Delta Squad brings some doubt to that theory. Another theory is that the Locusts have been underground long before the humans were on Sera, and that the Locusts are merely trying to protect Sera and wipe out the humans, who are slowly defacing Sera with wars and such.